Light source comprising a large number of light-emitting diodes

ABSTRACT

A light source is described, which is composed of a large number of light-emitting diodes ( 7 ) in an array arrangement. The light source comprises a mounting board ( 2 ) on which a flexible printed circuit ( 4 ) is adhesively bonded, on whose free surface a large number of light-emitting diodes ( 7 ) are mounted. Their lines ( 8 ) make contact with contact pads ( 10 ) at the ends of conductor tracks ( 9 ) on the flexible printed circuit ( 4 )

[0001] The invention relates to a light source comprising a large numberof light-emitting diodes.

[0002] In order to provide sufficient light intensities for indicators,displays, back lighting in vehicles, illumination for indicationinstruments, etc., it has already been proposed for a number oflight-emitting diodes to be combined to form what is referred to as anarray. In a light source such as this, the light-emitting diodes (LEDs)are located on the mount, which is also provided with a number ofcontact pads, which make electrical contact with the light-emittingdiodes.

[0003] A mount populated in such a way is placed, for example, on asolid printed circuit board, which is provided with equivalent contactpoints. In order to produce the electrical connection, connectionelements must be used between the contact pads on the mount and thecontact surfaces on the printed circuit, making electrically conductivecontact with the contact pads. Each connection thus has two junctionpoints, namely one on the mount and the other on the printed circuit,Furthermore, the light-emitting diodes must be connected to the contactpads on the mount. The wiring is thus very complex.

[0004] The invention is thus based on the problem of providing a lightsource which is of simple construction and can be integrated as easilyas possible in a light, under a display, in an optical system or in someother appliance configuration.

[0005] The invention therefore proposes that the light-emitting diodesbe mounted alongside one another on one face of a flexible printedcircuit, and the electrically conductively connected conductor tracks onthe flexible printed circuit.

[0006] This arrangement has the advantage that the light-emitting diodescan be connected directly to the current-carrying lines on the flexibleprinted circuit. This therefore reduces the number of connections to beproduced.

[0007] In order to make the arrangement mechanically robust, it isparticularly advantageous to mount the flexible printed circuit on arobust mount, which is at the same time used to dissipate the heatproduced by the light-emitting diodes. To this end, this mount ispreferably composed of a thermally conductive material, for examplecopper, and is possibly connected to a heat sink, or is in the form ofsuch a heat sink.

[0008] The flexible printed circuit on which the light-emitting diodesare mounted is connected to the mount in a simple manner by means ofthermally conductive adhesive.

[0009] The electrical connection between the light-emitting diodes andthe current-carrying lines on the flexible printed circuit is producedvia contact pads, with electrical contacts being made between the LEDsand the flexible printed circuit.

[0010] The light-emitting diodes can be integrated individually or as agroup of a number of them in the semiconductor chip, in which casesections of the chip can be doped appropriately (also differently).Furthermore, the semiconductor chip may have a corresponding number ofcontact pads, via which the light-emitting diode section or sections isor are supplied with power. When using such semiconductors, only thecontact pads on the chip need to be electrically conductively connectedto the corresponding contact pads on the flexible printed circuit.

[0011] This connection can be produced by soldering, bonding or adhesivebonding. The tern bonding refers to a specific welding process, which isknown per se, and which has been proven for populating printed circuitswith electronic components and which is carried out here specifically onthe pads on the flexible printed circuit.

[0012] In order to make the arrangement robust, the invention alsoproposes that the light-emitting diodes be arranged in an encapsulationcompound, which preferably extends to such an extent that only the lightoutlet surfaces remain free. This results in a mechanical, extremelyrobust arrangement. Power is supplied to the light-emitting diodes viathe conductor track on the flexible printed circuit, which projects outof the encapsulation compound at the side and can be connected to arigid printed circuit board via known connector systems.

[0013] The invention will be explained in more detail in the followingtext with reference to an exemplary embodiment. In the figures:

[0014]FIG. 1 shows a plan view of a light-emitting diode array,

[0015]FIG. 2 shows a section along the line II-II.

[0016] The light-emitting diode array 1 comprises a square mountingboard 2, preferably composed of copper. There are boreholes 3 in thecorners of the mounting board 2, using which the mounting board 2 can bemounted at a suitable position. A flexible printed circuit 4 isadhesively bonded to one side face, and has a square accommodation area5 and a supply line area 6 in the form of a strip. Up to 100light-emitting diodes 7 are adhesively bonded onto the accommodationarea 5, and are represented here only as small square surfaces. Theseare connected to the conductor tracks 9 via lines 8, of which only a feware illustrated. The connection is made via small contact pads 10, oneof which is illustrated schematically. The conductor track 9 and contactpads 10 are part of the flexible printed circuit 4. The illustration ofthe contact pad 10 and of the line 8, which is a thin wire composed ofaluminum or gold, is highly magnified.

[0017] The accommodation area 5 has a number of contact pads 10corresponding to the number of light-emitting diodes. These contact pads10 are arranged such that electrical contact can be made in a simplemanner. In order to supply power to the light-emitting diodes 7, thelines 8 are connected firstly to contact pads—which are not shown in anymore detail here because they are so small—on the light-emitting diodes,and to the contact pads 10 on the flexible printed circuit 4.

[0018] The bonding process has been particularly proven for connection,in which the lines 8 are welded to the contact pads 10 on the flexibleprinted circuit 4. This process can be used particularly well when it isnecessary to produce a large number of electrically conductive contactsin a very confined space.

[0019] The flexible printed circuit 4 which has been populated in thisway is adhesively bonded onto the mounting board 2 using a thermallyconductive paste. The area of the light-emitting diodes 7 is thensurrounded by an encapsulation compound 11, which provides furtherrobustness for the arrangement. As is illustrated schematically in FIG.2, the encapsulation compound 11 extends over the edge of the flexibleprinted circuit 4 and as far as the upper edge of the light-emittingdiodes 7, So that only the light outlet surfaces of the light-emittingdiodes 7 remain free. The encapsulation compound 11 makes thearrangement robust, and protects against damage.

[0020] As both figures show, the supply line area 6 on the flexibleprinted circuit 4 projects at the sides, A plug can be attached to itsfree end, so that it is possible to produce a connection to a socket ona rigid printed circuit board.

[0021] The arrangement has the advantage that a large number oflight-emitting diodes 7 can be supplied with power via a commonline—this being the supply line area 6 on the flexible printed circuit4. The process of making contact with the light-emitting diodes 7 isconsiderably simplified, since the lines 8 of the light-emitting diodes7 just need to be linked to contact pads 10 on the flexible printedcircuit 4. The thermally conductive mounting board 2 also results in theentire light source having a good thermal budget,

1. A light source comprising a large number of light-emitting diodes, characterized in that the light-emitting diodes (7) are mounted alongside one another on one face of a flexible printed circuit (4), and are electrically conductively connected to conductor tracks (9) on the flexible printed circuit (4).
 2. The light source as claimed in claim 1, characterized in that the flexible printed circuit (4) is mounted with that face which is opposite the light-emitting diodes (7) on a stable mounting board (2), for heat dissipation.
 3. The light source as claimed in claim 2, characterized in that the mounting board (2) is composed of thermally conductive material.
 4. The light source as claimed in claim 3, characterized in that the mounting board (2) is connected to a heat sink or is in the form of a heat sink.
 5. The light source as claimed in claim 2, characterized in that the flexible printed circuit board (4) is connected to the mounting board (2) by means of a thermally conductive adhesive or a thermally conductive adhesion layer.
 6. The light source as claimed in claim 1, characterized in that the conductor tracks (9) end in contact pads (10) on the flexible printed circuit (4), with the lines (8) which originate from the light-emitting diodes (7) making electrically conductive contact with the contact pads (10).
 7. The light source as claimed in claim 6, characterized in that one or more of the light-emitting diodes (7) is or are integrated in a semiconductor chip, and the semiconductor chip has a corresponding number of contact pads, which are electrically conductively connected to the corresponding contact pads (10) on the flexible printed circuit (4).
 8. The light source as claimed in claim 7, characterized in that the electrically conductive connections are produced via wires, which are connected to the contact pads on the flexible printed circuit and the semiconductor chip by soldering, bonding or adhesive bonding.
 9. The light source as claimed in one of the preceding claims, characterized in that the light-emitting diodes (7) are arranged in an encapsulation compound (11).
 10. The light source as claimed in claim 9, characterized in that the encapsulation compound (11) extends as far as the light outlet surface of the light-emitting diodes (7). 